Clustered Regulatory Interspaced Short Palindromic Repeats (CRISPR), and CRISPR-associated (cas) genes, referred to as CRISPR/Cas systems, are used by various bacteria and archaea to mediate defense against viruses and other foreign nucleic acid. These systems use small RNAs to detect and silence foreign nucleic acids in a sequence-specific manner.
Three types of CRISPR/Cas systems have been described (Makarova et al., Nat. Rev. Microbiol. 9, 467 (2011); Makarova et al., Biol. Direct 1, 7 (2006); Makarova et al., Biol. Direct 6, 38 (2011)). Recent work has shown that Type II CRISPR/Cas systems can be engineered to direct targeted double-stranded DNA breaks in vitro to specific sequences by using a single “guide RNA” with complementarity to the DNA target site and a Cas9 nuclease (Jinek et al., Science 2012; 337:816-821). This targetable Cas9-based system also works in cultured human cells (Mali et al., Science. 2013 Feb. 15; 339(6121):823-6; Cong et al., Science. 2013 Feb. 15; 339(6121):819-23) and in vivo in zebrafish (Hwang and Fu et al., Nat Biotechnol. 2013 March; 31(3):227-9) for inducing targeted alterations into endogenous genes.